Heart Valve Malfunction in the Dog (Mitral Insufficiency)

Many dogs slowly develop degenerative thickening and progressive deformity of one or more heart valves as they age. In time, these changes cause the valve to leak. The mitral valve is most commonly affected. This valve separates the blood collecting chamber (left atrium) from the pumping chamber (left ventricle) leading to the body. Some dogs also develop these changes in the tricuspid valve, which separates the collecting (right atrium) and pumping (right ventricle) chamber leading to the lungs.

Certain breeds have an inherited predisposition to this disease. Degenerative valvular disease is slowly progressive over years and is non-reversible. The volume of blood that leaks back into the atrium with each heartbeat tends to increase slowly over time. However, many dogs with this disease never develop signs of congestive heart failure even though they may have a loud murmur. Early in the disease process, your veterinarian may hear a soft murmur when the affected valve starts to leak. There usually is no noticeable change in the dog’s activity level or behavior for a long period of time. Gradually, though, the valve leak tends to get worse and the heart slowly enlarges. If the leak becomes severe, blood may start to back up behind the heart – usually into the lungs. This causes lung congestion and fluid accumulation (edema). When lung congestion and edema occur, congestive heart failure is present.

Reduced exercise ability may be the first sign of heart failure. Most dogs with heart failure caused by degenerative valve disease show signs of "left-sided" congestive failure. These signs include tiring quickly, increased breathing rate or effort for the level of activity, excessive panting, and cough (especially with activity). The presence of any of these signs should prompt a visit to your veterinarian to determine if heart failure (or another disease) has developed.

More advanced signs of heart failure could include labored breathing, reluctance to lie down, inability to rest comfortably, worsened cough, reduced activity, and loss of appetite. Your veterinarian should be consulted right away if these signs occur.

Some dogs that become symptomatic from their heart disease develop fluid in the abdomen (ascites); others have episodes of sudden weakness or fainting that can result from irregular heartbeats or other complications. As long as no sign of heart failure develops, no treatment is necessary, although reduction of dietary salt intake is often advised. Again, there are many dogs with degenerative valvular disease that never progress to heart failure.

However, if heart failure develops, several medications and other strategies are used to control the signs. Since the disease is not reversible and heart failure, when it occurs, tends to be progressive, the intensity of the therapy (including the number of medicines and dosages used) usually must be increased over time.